Method and apparatus for making submicrom powders

Abstract

A process for producing ultrafine particles includes the steps of providing a consumable electrode having a rod or wire configuration, melting the tip of the electrode by means of electron bombardment, and applying an intense electric field to the molten tip to generate a beam of charged droplets. By heating the tip of the rod but avoiding melting of the tip and subsequently applying an electric field to the heated tip, an ion beam may be generated.

What is claimed is: 
     
       1. A method of forming ultrafine particles comprising the steps of: providing a consumable rod of a material to be melted and formed into said particles, the rod having a diameter such that upon melting of the tip thereof the surface tension of the molten material will hold the molten tip against the unmelted portion of the rod;   melting the tip of the rod; and   applying an electric field to the molten tip to create electrostatic forces which are high enough to overcome the surface tension of the molten tip to thereby generate a beam of ultrafine droplets of said material.   
     
     
       2. The method of claim 1 including the step of solidifying the droplets to form a powder of ultrafine particles. 
     
     
       3. The method of claim 1 including the steps of: providing a workpiece having a surface to be coated; and   placing the surface of the workpiece in the path of the beam of droplets so that droplets will impinge upon the surface of the workpiece in their liquid state to thereby coat the surface of the workpiece.   
     
     
       4. The method of claim 1 wherein the step of melting the tip of the rod comprises the step of bombarding the tip of the rod with electrons. 
     
     
       5. The method of claim 1 wherein the electric field is maintained at a fixed location and including the step of feeding the rod of material as droplets are formed to maintain a desired positional relationship between the tip of the rod and the electric field. 
     
     
       6. The method of claim 1 wherein the steps of applying an electric field includes the steps of applying a positive voltage to the rod and providing a grounded extractor electrode near the tip of the rod. 
     
     
       7. The method of claim 1 including the step of heating a portion of the rod adjacent the molten tip to minimize heat conduction away from the tip through the rod. 
     
     
       8. A method according to claim 1 wherein the rod has a diameter of less than about 0.125 inches. 
     
     
       9. A method according to claim 1 wherein the rod has a diameter between about 0.02 inches and about 0.125 inches. 
     
     
       10. A method according to claim 1 wherein the method is carried out in a vacuum chamber. 
     
     
       11. A method according to claim 10 including the step of introducing a reactive gas into the vacuum chamber during the production of ultrafine particles, whereby said gas will react with the particles to form a desired material. 
     
     
       12. A method according to claim 1 wherein said rod of material is formed of plural distinct materials to be processed. 
     
     
       13. A method of forming ultrafine particles comprising the steps of: providing a sheet of a material to be melted and formed into said particles, the sheet having a thickness such that upon melting a lower edge of the sheet the surface tension of the molten material will hold the molten edge against the unmelted portion of the sheet;   melting a lower edge of the sheet; and   applying an electric field to the molten edge to create electrostatic forces which are high enough to form the molten material into a plurality of cones and overcome the surface tension of the molten cones to thereby generate beams of ultrafine droplets of said material.   
     
     
       14. An apparatus for producing ultrafine particles of material, comprising: means for supplying a solid piece of material from which particles are to be formed;   means for heating the material to melt a small portion thereof, said molten portion remaining attached to the remainder of the piece of material by surface tension; and   means for applying an electric field to the molten portion high enough to form the molten portion into a conical configuration and high enough to overcome the surface tension of the molten portion to thereby generate a beam of ultrafine droplets of said material.   
     
     
       15. An apparatus as in claim 14 wherein the means for heating includes an electron source for bombarding the portion of the material to be melted with electrons. 
     
     
       16. An apparatus according to claim 15 wherein the piece of material comprises a rod having a tip and the means for bombarding includes an annular electron emitter located near said tip. 
     
     
       17. An apparatus according to claim 15 wherein the electron emitter is a tantalum filament. 
     
     
       18. An apparatus according to claim 14 wherein the means for applying the electric field includes supply means for applying a high voltage to the piece of material and a grounded extractor electrode located near the molten portion. 
     
     
       19. An apparatus according to claim 14 including collector means for collecting the droplets after they have solidified. 
     
     
       20. An apparatus according to claim 19 wherein the collector means includes means for decelerating the droplets prior to collection. 
     
     
       21. An apparatus according to claim 20 wherein the droplets produced are charged droplets and wherein the means for decelerating includes means for generating an electric field in opposition to the direction of motion of the droplets. 
     
     
       22. An apparatus for producing ultrafine particles of material, comprising: means for feeding a rod of consumable workpiece material having a lower tip;   electron source means for bombarding the tip of the rod to melt the tip in a region of size such that the surface tension of the molten material holds the molten material against the unmelted portion of the workpiece; and   means for applying an electric field to the molten tip high enough to form a Taylor cone from the molten material and to overcome the surface tension of the cone to thereby generate a beam of ultrafine droplets of said material, said means including a grounded extractor electrode near the tip of the rod and a power supply for applying a high voltage to the rod.   
     
     
       23. An apparatus according to claim 22 wherein the power supply is operated in a current regulated mode.